The effects of ingestion of FAC (fructose, arginine, and citric acid combined) were compared with that of GAC (glucose, arginine, and citric acid combined) and placebo (treatment with no active beneficial effect) on metabolism of lipids and carbohydrates in seven male runners during prolonged exercise. As discussed in the study (Costill et al., 1977), the amount of glucose in the blood increases because of the intake of glucose before exercise and it indorses the metabolism of carbohydrates by promoting the secretion of insulin which then consumes glycogen in muscles. In the present study, when subjects ingested GAC during rest, the amount of glucose in plasma and serum insulin augmented, whereas after the intake of FAC and placebo, there was no change in the values. At the end of the prolonged exercise, the values of the glucose in plasma decreased for GAC. This explains as mentioned in (Costill et al., 1977), that liver glycogen was missing since during the prolonged exercise, the carbohydrate was promoted to be consumed as compared to the GAC intake. As discussed earlier, after the intake of FAC, there was no change in the values of plasma glucose during rest or in prolonged exercise, but the values of plasma glucose when ingested FAC remained higher than ingested placebo during exercise. As revealed in the studies (Nilsson and Hultman, 1974; Saitoh and Suzuki, 1986; Conlee et al., 1987), that fructose is mostly carried to the liver then as an assumption that as a source of energy, carbohydrates should be given continuously. The source of energy ratio, R value is the ratio of amount of expired carbon monoxide to the amount of oxygen. These studies (Levine et al., 1983; McMurray et al., 1983; Massicotte et al., 1986; Guezennec et al., 1989; Murray, 1989) suggest that R value relates to how the amount of glucose, free fatty acids and glycerol alter in blood during prolonged exercise. Not only that these studies explain that the decline in R value is related with the elevation of the free fatty acids in the blood after ingestion of fructose. In the present study, the R value decreased after the ingestion of FAC rather than the intake of GAC during exercise. The results of this study propose that before and during the prolonged exercise, the intake of fructose is beneficial as it is a source of energy without lipolysis inhibition and must be supplied continuously.